The present invention relates in general to information storage media, and more particularly, to holographic data storage systems (HDSS). More specifically, this invention relates to methods and devices for associative, or content-addressable, data retrieval. The invention can also be applied to a device that does not support associative retrieval.
Holographic data storage is accomplished by intersecting two coherent light beams in a photosensitive medium in order to record a hologram. Data retrieval is accomplished by illuminating the hologram with a replica of one of the beams, thereby causing a replica of the other beam to be reconstructed through the physics of holography. In the case of sufficiently thick media, a plurality of holograms (a hologram xe2x80x9cstackxe2x80x9d) may be recorded in the same location (or in overlapping locations).
A hologram is a volume or film of photosensitive material that records the interference pattern of two light sources. To create a hologram, laser light is first split into two beams, an object beam and a reference beam. The object beam is then manipulated and sent into the photosensitive material. Once inside this material, it intersects the reference beam and the resulting interference pattern of laser light is recorded in the photosensitive material, resulting in a hologram. Once a hologram is recorded, it can be viewed with the reference beam alone. The reference beam is projected into the hologram at the exact angle it was projected during recording. When this light hits the recorded diffraction pattern, the object beam is regenerated out of the diffracted light. An exact copy of the object beam is sent out of the hologram and can be read by optical sensors. For example, a hologram that can be obtained from a toy store illustrates this idea. Precise laser equipment is used at the factory to create the hologram. A recording material that can recreate recorded images out of natural light is used so the consumer does not need high-tech equipment to view the information stored in the hologram. Natural light becomes the reference beam and human eyes become the optical sensors. Generally, white light holograms use a variation in the recording process, not special media.
In order for holographic technology to be applied to computer systems, it must store data in a form that a computer can recognize. In current computer systems, this form is binary. In the previous section, it was mentioned that the source beam is manipulated. In common holograms, this manipulation is the creation of an optical image such as a ball or human face. In computer applications, this manipulation is in the form of bits. The next section explains the spatial light modulator, a device that modulates laser light with binary data.
Typically in the prior art, the two beams are assigned distinct roles. The xe2x80x9cobject beamxe2x80x9d is modulated in some manner (e.g., with a spatial light modulator, or xe2x80x9cSLMxe2x80x9d) that allows it to carry data. A spatial light modulator is used for creating binary information out of laser light. The SLM is a 2D plane, consisting of pixels that can be turned on and off to create binary 1""s and 0""s. An illustration of this is a window and a window shade. It is possible to pull the shade down over a window to block incoming sunlight. If sunlight is desired again, the shade can be raised. A spatial light modulator contains a two-dimensional array of xe2x80x9cwindowsxe2x80x9d which are only microns wide. These windows block some parts of the incoming laser light and let other parts go through. The resulting cross section of the laser beam is a two dimensional array of binary data, exactly the same as what was represented in the SLM. This is correct right in the plane of the SLM, but as the beam propagates diffraction could cause the cross section to evolve into other shapes. After the laser beam is manipulated, it is sent into the hologram to be recorded. This data is written into the hologram as page form. It is called this due to its representation as a two dimensional plane, or page, of data.
The other beam, the xe2x80x9creference beam,xe2x80x9d is drawn from an enumerable set of possible reference beams (e.g., plane waves incident at differing angles) designed to have characteristics favorable to the holographic recording process. A plurality of data-bearing object beams may be recorded in the same volume of medium and retrieved independently provided that each is paired with a distinct reference beam during recording.
Holographic storage media take advantage of the photorefractive effect described by David M. Pepper et al., in xe2x80x9cThe Photorefractive Effect,xe2x80x9d Scientific American, October 1990 pages 62-74. The index of refraction in photorefractive materials can be changed by light that passes through them. By controllably changing the index of refraction in such materials, information can be stored in the photorefractive material in the form of interference patterns (or holograms). Holographic storage systems allow for high-density, high-capacity, and high-speed storage of information in photorefractive and photopolymers (or holographic) storage media.
A hologram stores data in three dimensions and reads an entire page of data at one time, which is unlike an optical CD disk that stores data in two dimensions and reads one bit at a time. The advantages of recording a hologram are high density (storage of hundreds of billions of bytes of data), high speed (transfer rate of a billion or more bits per second) and ability to select a randomly chosen data element in 100 microseconds or less. These advantages arise from three-dimensional recording and from simultaneous readout of an entire page of data at one time.
A hologram is a pattern, also known as a grating, which is formed when two laser beams interfere with each other in a light-sensitive material (LSM) whose optical properties are altered by the intersecting beams. Before the bits of data can be imprinted in this manner in the LSM, they must be modulated by a SLM to be represented as a pattern of clear and opaque squares on a display such as a liquid crystal display (LCD) screen, a miniature version of the ones in laptop computers. A blue-green laser beam, for example, is shined through this crossword puzzle-like pattern called a page, and focused by lenses to create a beam known as an object beam. A hologram of the page of data is created when the object beam meets another beam, called the reference beam, in the LSM. The reference beam could be collimated, which means that all its light rays propagate in the same direction. The term for xe2x80x9csynchronizedxe2x80x9d light is xe2x80x9ccoherent,xe2x80x9d and coherence is necessary for holography. Such waves are known as plane waves. The grating created when the signal and reference beams meet is captured as a pattern of varying refractive index in the LSM.
After recording the grating, the page can be holographically reconstructed by shining the reference beam into the LSM from the same angle at which it had entered the LSM to create the hologram. As it passes through the grating in the LSM, the reference beam is diffracted in such a way that it recreates the original object beam and the information contained on it. The reconstructed object beam is then focused into an image of the original page onto a detector such as an array of electrooptical detectors that sense the light-and-dark pattern, thereby reading all the stored information on the page at once. The data can then be electronically stored, accessed or manipulated by any conventional computer.
As explained above, in the typical holographic storage system, two coherent light beams are directed onto a photosensitive storage medium. The first coherent light beam is an object beam, which is used to encode data. The second coherent light beam is a reference light beam. The two coherent light beams intersect within the storage medium to produce an interference pattern. The photosensitive storage medium records this interference pattern by changing its index of refraction to form a diffraction grating.
The recorded information, stored as a hologram, can be read by illuminating the hologram with a reference beam. A hologram is not really an image, though it is often referred to as an xe2x80x9cimage.xe2x80x9d When the hologram is illuminated with a reference beam at an appropriate angle, an object beam containing the information stored is produced. The resulting object beam is then typically focused onto a sensor such as a Charge Coupled Device (CCD) array or an active pixel sensor. The sensor is attached to a decoder, which is capable of decoding the data.
More than one hologram may be stored in the same volume by xe2x80x9cmultiplexing,xe2x80x9d for example by varying the angle of the reference beam during recording. Accordingly, high storage capacity can be obtained since the same volume can be used to store multiple holographic recordings.
For a typical data retrieval operation, a replica of the reference beam is used to illuminate the hologram, and a replica of the object beam is reconstructed. Detecting and decoding the modulated object beam thereby allows the retrieval of the stored data.
A conventional read head reads one bit at a time, a holographic read head reads one page at a time. Because data is stored as page data in a hologram, the retrieval of this data may also be in this form. Page data access is the method of reading stored data in sheets, not serially as in conventional storage systems. Holographic memory reads data in the form of pages.
In principle, the holographic physics do not distinguish between the signal and reference beams. Thus, the converse operation is also possible. If a replica of an object beam illuminates a hologram stack, then a replica of the reference beam used to record that object beam will be reconstructed. If several identical object beams have been recorded with different reference beams, then each of those reference beams will be simultaneously reconstructed. Furthermore, if a probe object beam (xe2x80x9cprobe beamxe2x80x9d) that is not necessarily identical to any of the recorded object beams is used, then the reference beams associated with every object beam in the hologram stack will be simultaneously reconstructed with intensity in proportion to the correlation coefficient of the probe beam with the recorded object beam. Then, the reconstructed reference beams could be detected, indicating the address of the desired recorded object beams.
More particularly, the associative holographic data retrieval is has the following features. The amount of power diffracted into each xe2x80x9coutputxe2x80x9d beam is proportional to the 2D cross-correlation coefficient between the input data page of the probe beam and the stored data page (previously recorded with that particular reference beam). Each set of output beams can be focused onto a detector array, so that each output beam forms its own correlation xe2x80x9cpeak.xe2x80x9d The center of each correlation peak represents the simple overlap between the input data page of the probe beam and the associated stored page in the holographic recording medium. The term xe2x80x9ccoefficientxe2x80x9d above is used to distinguish from a correlation xe2x80x9cfunction,xe2x80x9d which is evaluated at all possible offsets between the two functions. In this invention, the optical correlation process merely provides the 2-D cross-correlation coefficient. When the input data page of the probe beam contains patterns that correspond to some data fields stored in the holographic recording medium, the optical correlation process allows the simultaneous querying of a plurality of storage locations for the presence of the given data patterns.
This is one basis for associative holographic data retrieval as contemplated in the prior art. By creating a probe beam that contains a datum or data of interest, a holographic storage device can query an entire stack of holograms simultaneously and locate those data pages that are highly correlated with the probe beam. These are the data pages that will (probably) contain the datum of interest.
It is the associative holographic data retrieval that gives content-addressable holographic data storage an inherent speed advantage over a conventional serial search, especially for large databases. For instance, if a conventional hard-disk drive has a readout rate of 25 MB/s, a search over one million 1 KB records would take about 40 s. In comparison, a holographic system could search the same records in about 30 ms, which is 1200xc3x97 improvement.
A holographic storage medium includes the material within which a hologram is recorded and from which an image is reconstructed. A holographic storage medium may take a variety of forms. For example, it may comprise a film containing dispersed silver halide particles, photosensitive polymer films (xe2x80x9cphotopolymersxe2x80x9d) or a freestanding crystal such as iron-doped LiNbO3 crystal. U.S. Pat. No. 6,103,454, entitled RECORDING MEDIUM AND PROCESS FOR FORMING MEDIUM, generally describes several types of photopolymers suitable for use in holographic storage media. The patent describes an example of creation of a hologram in which a photopolymer is exposed to information carrying light. A monomer polymerizes in regions exposed to the light. Due to the lowering of the monomer concentration caused by the polymerization, monomer from darker unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient creates a refractive index change forming a hologram representing the information carried by the light.
FIG. 1 illustrates the basic components of a holographic system 100. System 100 contains a SLM 112, a holographic storage medium 114, and a sensor 116. SLM 112 encodes beam 120 with an object image. The image is stored by interfering the encoded object beam 120 with a reference beam 122 at a location on or within holographic storage medium 114. The interference creates an interference patterns (or hologram) that is captured within medium 114 as a pattern of, for example, a holographic refractive index grating.
It is possible for more than one holographic image to be stored at a single location, or for a holographic image to be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 122, depending on the particular reference beam employed. Object beam 120 typically passes through lenses 130 before being intersected with reference beam 122 in the medium 114. It is possible for reference beam 122 to pass through lenses 132 before this intersection. Once data is stored in medium 114, it is possible to retrieve the data by intersecting a reference beam 122 with medium 114 at the same location and at the same angle, wavelength, or phase at which a reference beam 122 was directed during storage of the data. The reconstructed data may pass through one or more lenses 134 and is detected by sensor 116. Sensor 116 is, for example, a charged coupled device or an active pixel sensor. Sensor 116 typically is attached to a unit that decodes the data.
The quality of the recorded hologram as measured by such parameters as diffraction efficiency, multiplexing selectivity, and image fidelity is directly influenced by a variety of details specific to each system implementation. U.S. Pat. No. 5,416,616 (Jenkins) makes a statement to the effect of measuring the xe2x80x9cfidelityxe2x80x9d of a holographic copy.
U.S. Pat. No. 6,175,543 (Burr) contains a variety of techniques to improve the performance of content-addressable holographic memories. It contemplates modifying the reference beam, organizing the data patterns and conditioning the correlation signal. All these techniques are purported to improve the correlation signal-to-noise ratio during an associative read or search operation.
U.S. Pat. No. 5,754,691 (Hong), like Burr, discloses methods for improving the performance of a holographic correlator. In particular, it describes a process for normalizing the correlation strengths by the overall hologram strengths in order to more accurately establish the true degree of correlation. By contrast, for xe2x80x9cAssociative Write Verifyxe2x80x9d the degree of correlation is (in principle) already known, and it is the actual hologram strength that is determined.
U.S. Pat. No. 5,671,090 (Pernick) discusses the use of an optical correlation to search a database for a given xe2x80x9csequence,xe2x80x9d particularly for searching genetic databases for DNA sequence matches. This is a traditional application of a content-addressable memory that does not address the fundamental role of xe2x80x9cAssociative Write Verify,xe2x80x9d that is, data integrity verification in a storage device.
U.S. Pat. No. 5,416,616 (Jenkins) discloses a technique for copying multiple holograms simultaneously using multiple light sources that are self-coherent but mutually incoherent. U.S. Pat. No. 4,860,253 (Owechko) addresses practical concerns for implementing a content-addressable holographic memory. When a partial and/or distorted image is presented as a search key to an associative memory, the result of the optical correlation is a partial and/or distorted version of the reference beam(s) used to write those records that correlate well with the search key. The patent discusses feedback methods for deriving a clean version of this reference beam(s) so that the associated records may be recovered cleanly. It does not seem to address optical correlation as a write verification tool. U.S. Pat. No. 3,657,473 (Corcoran) is an old patent that describes a method of recording analog video holographically mentioning about interchanging the roles of the reference and object beams for optical correlation, but does not mention using this property for data verification.
In short, the prior art references relate to associative retrieval for the function of content-addressable memory during read operations. Nowhere do the prior art references appear to contemplate the use of an associative verification step for a write operation in order to assure data integrity, which is a feature of this invention. Furthermore, the implementation of a content-addressable memory is considerably more complex than what is required to implement the associative write verify method of this invention.
An embodiment of this invention is an associative write verify system for a holographic recording medium, comprising (1) a hologram, (2) an object beam, (3) a reference beam, (4) a probe beam, (5) a reconstituted reference beam and (6) means for comparing the reference beam with the reconstituted reference beam, wherein the system implements an associative write verify during holographic recording. The system could further comprise a stored address with a one-to-one correspondence to the reference beam and a code comprising a data pattern within the object beam. The probe beam could be modulated to match the code. The means for comparing the reference beam with the reconstituted reference beam could comprise means for detecting the reconstituted reference beam. The associative write verify could be selected from the group consisting of a parallel associative write verify, a post-glimpse page-wise verify and combinations thereof. The stored address could correspond to a hologram page or a reference beam angle used to record a hologram page. The stored address could be stored in a microprocessor RAM memory or in a portion of the holographic recording medium. The code could be a pattern in a hologram page or information from different hologram pages. The probe beam could be generated by an object beam modulator. The reference beam could be a plane wave reference beam generated using scanning mirrors or an array of laser beam generators. The reconstituted reference beam could be collected by a lens. The means for comparing the reference beam with the reconstituted reference beam could comprise a hardware or software comparator.
The system could further comprise means for associative, post-glimpse page-wise verify. The means for associative, post glimpse page-wise verify could comprise detection of the reconstituted reference beam while an original data pattern remains on a modulator after writing.
The holographic recording medium could be an optically flat planar medium. The reconstituted reference beam could be detected with a photo detector.
In one variation, the holographic recording medium is a holographic recording medium comprising a polymer matrix. The holographic recording medium could have a Rayleigh ratio (R90xc2x0) of less than 7xc3x9710xe2x88x923 cmxe2x88x921. The holographic recording medium could have a thickness greater than 200 xcexcm and a refractive index contrast (xcex94n) of 3xc3x9710xe2x88x923 or higher.
In one variation, the system could have a probe pattern and/or geometry capable of detecting and identifying the reconstituted reference beam. The probe pattern and/or geometry for an angular multiplexing system could comprise a lens to intercept the reconstituted reference beam and focus the reconstituted reference beam into a resolvable spot on a detector array, the reconstituted reference beam being a plane wave. The probe pattern and/or geometry could be capable of collecting the reconstituted reference beam with a lens and imaging an origin of the spherical beam onto a detector, the reconstituted reference beam being a spherical beam. The probe pattern and/or geometry could comprise optical elements for separating the propagating modes of the reconstituted reference beam into separated mode reconstituted reference beams and resolving said separated mode reconstituted reference beams onto a detector. The code could comprise one or more substantially mutually orthogonal modulation codes for marking a copyright status of data within a page recorded in the holographic recording medium.
Another embodiment is a method for associative write verify system for a holographic recording medium, comprising (1) interfering an object beam with a reference beam onto the holographic recording medium to form a hologram; (2) shining a probe beam; (3) forming a reconstituted reference beam and (4) comparing the reference beam with the reconstituted reference beam, wherein the method implements an associative write verify during holographic recording.
The method of could further comprise impinging the reconstituted reference beam upon a holographic optical element, wherein the reconstituted reference beam is recorded with a corresponding collimated or spherical index beam. The reconstituted reference beam could be reconstituted in a system comprising wavelength, phase code or correlation multiplexing. The method could further comprise impinging the reconstituted reference beams upon a grating or prism whereby individual reconstituted reference beams of differing wavelength are separated. The method could further comprise marking a copyright status of data within a page recorded in the holographic recording medium and determining whether the data is under copyright restriction. The page could be recorded before marking the copyright status of the data. The marking could be done in an area of a disk wherein substantially no user data is stored. The area could be a format area.
As will be realized, this invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.